OH + H2 → H2O + H. The importance of ‘exact exchange’ in density functional theory

The potential energy surface of the title reaction has been studied at HO … HH distances between 1.0 and 2.1 Å along a Cs reaction path with a variety of density functionals, both local and nonlocal, and compared to potential energy curves obtained at the Hartree-Fock, MP2 and CCSD(T) levels. Whereas the latter curves are repulsive from 2.1–1.4 Å with energies above that of the reactants, nearly all the DFT curves are either essentially flat or attractive with energies below the reactants, giving an entirely incorrect picture of this important radical reaction. Only the ACM functional — which includes part of the exact Hartree-Fock exchange — gives a definite nonzero barrier.